Gao et al. (2005) found the max turn over rate (Vmax) of the Na/K pumps is spatially regulated across the canine ventricular wall, with Vmax being largest in the epicardium (Epi), intermediate in the midmyocardium (Mid) and smallest in the endocardium (Endo). Moreover, this study reported the transmural gradient in Vmax creates a gradient in [Na+]j, which is highest in Endo, intermediate in mid and lowest in Epi. Our preliminary data suggest the gradient in [Na+]j alters Na/Ca exchange (NCX) to generate a transmural gradient in [Ca2+]( and contraction. Preliminary data also suggest this transmural gradient is generated through angiotensin II (A2), which appears to be regulated by mechanical feedback, possible by the titin kinase (TiK), which is a mechanical sensor. We have created a quantitative model relating Na/K pump current (IP), lNCx, [Ca2*]j and contraction. The model successfully predicts differences in contraction and Ca-transients recorded in myocytes from Endo and Epi. Based on these data, we hypothesize: 1) The purpose of the transmural gradient in Vmax is to generate maximum contractility and rate responsiveness in Endo, which contracts first and against the greatest pressure load during each action potential (AP). 2) This is a feedback control system in which the sensor is the TiK. Pressure dependent activation of TiK causes up regulation of an autocrine renin angiotensin system (RAS) that sets the gradient in max IP such that [Ca2+]j and hence contractility offsets transmural gradients in load. The aims of this proposal are to test these two hypotheses. We will use a combination of electrophysiological, Na- and Ca-imaging, contraction, and biochemical measurements to accomplish these aims.